1. Field of the Invention
This invention is directed to a control circuit for an automatic light adjusting circuit for an electronic flash device. More particularly, this invention is directed to a control circuit for detecting and controlling the light output from a flash lamp or the like. Specifically, this invention is directed to a current converting circuit for converting a current value of a signal from a photodetector into a logic-level voltage pulse train signal.
2. Description of Related Art
Heretofore, photometric circuits, provided with an electronic flash device and automatically adjusting the light output of the flash device, charged an integral capacitor, either directly through the photoelectric current of the light receiving element or indirectly through an amplifier. The known photometric circuits then compare the voltage level of the charged capacitor with a base value corresponding to the correct amount of light, using an analog comparator. These known photometric circuits are structured to automatically adjust the light output by turning off the light-emitting element once the correct amount of light has been generated.
In these known photometric circuits, a large percentage of the circuitry is formed by analog circuits, including a light-receiving amplifier, a photoelectric current integrated circuit, a comparator and the base voltage supply circuit. For this reason, interfacing was difficult when controlling the analog photometric circuit with digital circuits such as microcomputers. In these known photometric circuits, the gain of the amplifier and the base voltage are generated using a microcomputer D/A converter, but controlling the majority of the automatic light adjusting circuit's components requires analog circuits. Therefore, the scale of analog circuitry cannot be reduced, in comparison with the size of electronic flash devices before microcomputers were built into cameras. Quite to the contrary, by installing a microcomputer, the circuitry of the known electronic flash device becomes more complex and leads to a larger device and increased production costs.
It is no longer possible to design a camera without microcomputers because of the trend toward high performance products that include features such as electronic flash devices. Therefore, this invention aims to reduce the overall scale of circuitry by replacing the known analog circuitry with digital circuitry using microcomputers.
In order to process the photometric integral value with digital circuits such as a microcomputer, one first must convert the analog signals to digital signals. However, A/D circuits have a significant output delay compared to devices having a quick response time, such as the flash lamp of the electronic flash device. There are instances when the flash time necessary to output a small amount of light is below 50 microseconds. Under present conditions, a sequential comparative-type 8-bit A/D converter with an installed microcomputer is a speedy component, even including the setting and reading processing time. Even so, the A/D conversion using the fast converter takes around 20 microseconds. In this case, even if the light adjusting circuit determines the correct amount of light that has been output, because the A/D conversion cycle is incomplete relative to the flash time, a time lag occurs between the correct termination time and the time when the microcomputer turns off the flash lamp. Because of this, the light adjustment characteristic during low light output amount events is extremely poor. The conversion time can be shortened by using a flash A/D converter. However, the microcomputer is constantly comparing the A/D conversion results with the base values and must determine whether or not to turn off the flash lamp. During this period the microcomputer is completely unable to perform other operations. Therefore, conventional A/D conversion methods are unsuitable and a new method is necessary.
One new method creates a number of logic level voltage pulses in proportion to the photoelectric current from the light-receiving element and detects the amount of light output by the flash lamp by counting the pulses.